1. Field of the Invention
The present invention relates to the technical field of signal detection and, more particularly, to a method and device for detecting defect signals on an optical disc.
2. Description of Related Art
Currently, with the rapid development of information industry and the widespread use of personal computers, people relatively increase on the requirement of data storage capability. Due to having large storage capacity, optical discs are thus widely used for storing data. The optical discs include CDROM/R/RW discs, DVDROM SL/DL discs, DVD-R/-R9/-RW/-RAM/Download discs, DVD+R/+R9/+RW discs, BDROM SL/DL discs, BD-R SL/DL discs, BD-RE SL/DL discs, BD LTH discs and so on. The optical discs can offer the internal data better protection to avoid various damages.
However, for data storage, the aforementioned feature does not mean that the optical discs are a perfect storage medium. Some defects, such as deep and shallow scratches, even a fingerprint, can be present on the optical discs. Such defects can cause the system read and write errors and the interference, and thus detecting the defects on the optical discs is important for protecting the system from being interfered or unstable.
One way in the prior art essentially uses a main or side beam signal or the combination to detect the defects on the optical discs.
Another way in the prior art uses the difference of signal amplitude such as an RF level (RFLVL) to detect an existing defect. FIG. 1A is a schematic graph of signals of a deep defect detected by applying a well-known RF level detection. As shown in FIG. 1A, the RF signal 110 has a depressed portion 112 in the period 120, which indicates the data corresponding to the depressed portion 112 is damaged due to a defect, so the RF signal 110 in the period 120 cannot be read out. The depressed portion 112 indicates a defective region with a low reflectivity. This is commonly referred to as a black-dot defect, and the RF signal 110 in the depressed portion 112 almost disappears.
Further, the depth of the depressed portion 112 indicates the defect depth. The RF level signal 114 RFLVL produced by passing the RF signal 110 through a low pass filter is the envelope of the RF signal 110. The detection threshold 130 can be a fixed DC reference voltage. When the RFLVL signal 114 in the period 120 is lower than the detection threshold 130, a detect flag signal 140 is changed from a low level (logic 0) to a high level (logic 1) to thereby indicate that a defect is detected. In addition, a focusing error/tracking error (FE/TE) signal 150 has a positive surge 152 at the start of the period 120 to indicate a focusing error and a negative surge 154 at the end of the period 120 to indicate a tracking error. However, when the defect flag signal 14 is changed from the low to the high level, the servo system such as an FE/TE servomotor, and the data path control system such as a preamplifier, data slicer or phase lock loop (PLL) can know the situation that the defect signal is detected. Accordingly, some appropriate protection methods and devices can be used to reduce the voltage interference and instability.
FIG. 1B is a schematic graph of signals of a shallow defect detected by applying a well-known RF level detection. As shown in FIG. 1B, the RF signal 101-1 has a depressed portion 112-1 in the period 120-1. This also indicates that the data corresponding to the depressed portion 112-1 is damaged due to a defect, and accordingly the RF signal 110-1 in the period 120-1 cannot be read out at all. However, the depth of the depressed portion 112-1 may be affected just by a shallow defect such as a shallow scratch that is not as deep as the depressed portion 112 of FIG. 1A. The RFLVL signal 114-1 shows the envelop of the RF signal 110-1. The detection threshold 130-1, like the detection threshold 130 of FIG. 1A, can be a programmable DC reference voltage. Obviously, the RFLVL signal 114-1 always is higher than the detection threshold 130-1 because the concave portion 112-1 caused by the shallow detect is not deep enough. Therefore, the defect flag signal 140-1 does not respond to the shallow defect, and in this case noises can be introduced easily to thereby produce a defect and a decision mistake if the DC reference voltage corresponding to the detection threshold is increased for detecting a shallow concave defect. The FE/TE signal 150-1 in the period 120-1 can be changed due to a disk defect's interference, so as to produce a false signal variation. Further, since the shallow detect is not detected, some protection methods and devices are not triggered to thereby protect the system from voltage interference and instability. Namely, the servo system and the data path control system can be affected easily by such a defect.
U.S. Pat. No. 7,301,871 granted to Lai, et al. for a “Method and device for detecting the signal on a disc having a defect by utilizing an RF signal and its derivatives” uses a defect detection unit to receive a plurality of defect signals and to set a plurality of defect flag signals and uses a logic combination unit to perform a logic operation on the defect flag signals to thereby detect a special defect. However, such a method only uses the logic combination unit to perform a determination on the existing defect signals, which cannot provide further defect information.
FIG. 2 is a schematic graph of a typical control signal detection of an optical disk drive. As shown in FIG. 2, the signal 210 is a tracking error signal (TE) outputted by a preamplifier, the signal 220 is an internal tracking error signal of a defect detection unit, the signal 230 is a tracking control signal (TRO) for driving a tracking actuator (TA) of a servo control unit, and the signal 240 is an RF signal (RF) outputted by the preamplifier. When a laser beam passes through a shallow scratch or fingerprint on a track of a disc, as shown at the circle A of FIG. 2, the tracking error signal 220 produces a surge to indicate that the laser beam passes through a defect, so the tracking control signal 230 produces a corresponding surge (at the circle B) to drive the tracking actuator to compensate the tracking error. However, in inspecting the waveform of the RF signal 240 (at the circle C), it is found that the waveform of the RF signal 240 is different from the black-dot defect in which almost no RF signal is presented, but meanwhile the tracking control signal 230 compensates the tracking error signal 220. The laser beam is originally at the track, but the laser beam after the compensation by the tracking control signal 230 slides to a neighboring track.
Therefore, lots of problems are still existed in the conventional skill for detecting defect signals on an optical disc, and thus it is desirable to provide an improved device and method to mitigate and/or obviate the aforementioned problems.